1. Field of the Invention
This invention relates to a transparent electro-conductive structure having a transparent substrate and formed successively thereon a transparent electro-conductive layer and a transparent coat layer, which is used in, e.g., front panels of display devices such as CRTs. More particularly, this invention relates to a transparent electro-conductive structure having superior weatherability, conductivity and so forth and also achievable of cost reduction in manufacture, and a process for its production, and also relates to a transparent electro-conductive layer forming coating fluid used for its production, and a process for preparing such a coating fluid.
2. Description of the Related Art
With office automation made in recent years, a variety of office information instruments have been introduced into offices, and in an office environment it is no longer uncommon to do office work all day while facing display devices of office information instruments.
Now, in office work done sitting close to cathode ray tubes (CRTs) of computers, as an example of the office information instruments, it is required for the display screens to be easy to watch and not to cause visual fatigue, as well as to be free from attraction of dust and electric shock which are due to the electrostatic charging on the CRT surfaces. Moreover, in addition to these, any adverse influence on human bodies by low-frequency electromagnetic waves generated from CRTs has been recently a concern, and it is desired for such electromagnetic waves not to leak outside.
The electromagnetic waves are generated from deflecting coils and flyback transformers and a large quantity of electromagnetic waves increasingly tend to leak to surroundings as CRTs become larger in size.
Now, the leakage of magnetic fields can be prevented to a great extent by designing, e.g., by the changing of deflecting coils in terms of shape. As for the leakage of electric fields, it can be prevented by forming a transparent electro-conductive layer on the front-glass surface of a CRT.
Measures to prevent such leakage of electric fields are theoretically the same as the countermeasures taken in recent years to prevent electrostatic charging. However, the transparent electro-conductive layer is required to have a much higher conductivity than any conductive layers formed for preventing the electrostatic charging. More specifically, a layer with a surface resistance of about 10.sup.8 ohm per square is considered sufficient for the purpose of preventing electrostatic charging. However, in order to prevent the leakage of electric fields (i.e., electric-field shielding), it is necessary to form at least a transparent electro-conductive layer with a low resistance of 10.sup.6 ohm per square or below, and preferably 10.sup.3 ohm per square or below.
Under such circumstances, as countermeasures for such a necessity, some proposals have been made. In particular, as a method that can attain a low surface resistance at a low cost, a method is known in which a transparent electro-conductive layer forming coating fluid prepared by dispersing conductive fine particles in a solvent together with an inorganic binder such as an alkyl-silicate, is coated on a front glass for a CRT, followed by drying and then baking at a temperature of 200.degree. C. or below.
This method making use of such a transparent electro-conductive layer forming coating fluid is much simpler than any other transparent electro-conductive layer forming method employing vacuum evaporation (vacuum deposition), sputtering or the like, and can enjoy a low production cost. Thus, it is a very advantageous method for electric-field shielding that can be applied to CRTs.
As the transparent electro-conductive layer forming coating fluid used in this method, a coating fluid is known in which indium tin oxide (ITO) is used as the conductive fine particles. Since, however, the resultant film has a surface resistance of as high as 10.sup.4 to 10.sup.6 ohm per square, a corrective circuit for cancelling electric fields is required in order to sufficiently shield the leaking electric fields. Hence, there has been a problem of a production cost which is correspondingly rather high. Meanwhile, in the case of a transparent electro-conductive layer forming coating fluid making use of a metal powder as the conductive fine particles, the resultant film may have a little lower transmittance than in the case of the coating fluid making use of ITO, but a low-resistance film of from 10.sup.2 to 10.sup.3 ohm per square can be formed. Accordingly, such a coating fluid, which makes the corrective circuit unnecessary, is advantageous in cost and is considered to become prevailing in future.
Fine metal particles used in the above transparent electro-conductive layer forming coating fluid, as disclosed in Japanese Patent Laid-open Application No. 8-77832 and No. 9-55175, are limited to noble metals such as silver, gold, platinum, rhodium and palladium, which may hardly be oxidized in air. This is because, if fine particles of a metal other than noble metals as exemplified by iron, nickel or cobalt are used, oxide films are necessarily formed on the surfaces of such fine metal particles in the atmosphere, making it impossible to attain a good conductivity in the transparent electro-conductive layer.
From another aspect, in order to make display screens easy to watch, an anti-glare treatment is conducted on the surfaces of face panels so that the screens can be restrained from reflecting light. This anti-glare treatment can be made by a method in which a finely rough surface is provided to make a diffused reflection on the surface greater. This method, however, can not be said to be preferable because its employment may bring about a low resolution, resulting in a low picture quality. Accordingly, it is preferable to make the anti-glare treatment by an interference method in which the refractive index and layer thickness of a transparent film are controlled so that the reflected light may interfere destructively with the incident light. In order to attain the effect of low reflection by such an interference method, it is common to employ a film of double-layer structure formed of a high-refractive-index film and a low-refractive-index film each having an optical layer thickness set at 1/4.lambda. and 1/4.lambda., or 1/2.lambda. and 1/4.lambda., respectively (.lambda.: wavelength). The film formed of fine particles of indium tin oxide (ITO) as mentioned above is also used as a high-refractive-index film of this type.
In metals, among parameters constituting an optical constant n-ik (n: refractive index; i.sup.2 =-1; k: extinction coefficient), the value of n is small but the value of k is extremely greater than that in ITO, and hence, also when the transparent electro-conductive layer formed of fine metal particles is used, the effect of low reflection that is attributable to the interference of light can be attained by the double-layer structure as in the case of ITO (a high-refractive-index film).
Now, as stated above, fine metal particles used in the conventional transparent electro-conductive layer forming coating fluid are limited to noble metals such as silver, gold, platinum, rhodium and palladium. To compare electrical resistance of these, platinum, rhodium and palladium have a resistivity of 10.6, 5.1 and 10.8 .mu..OMEGA..multidot.cm, respectively, which are higher than 1.62 and 2.2 .mu..OMEGA..multidot.cm of silver and gold, respectively. Hence, it has been advantageous to use fine silver particles or fine gold particles in order to form a transparent electro-conductive layer having a low surface resistance.
The use of fine silver particles, however, may cause a great deterioration due to sulfidation or contact with brine to cause a problem of weatherability. On the other hand, the use of fine gold particles can eliminate the problem on weatherability, but has had a problem on cost as in the case when fine platinum particles, fine rhodium particles or fine palladium particles are used. Moreover, the use of fine gold particles also has a problem that, because the transparent electro-conductive layer formed absorbs a part of visible light rays in itself because of the optical properties inherent in gold, the film can not be used in the display surfaces of display devices such as CRTs where flat transmitted-light profiles are required over the whole region of visible light rays.